1. Field of the Invention
The present invention relates to a device and method for reproducing images and, more specifically, to an image reproduction system whereby an image on the basis of digital image data which describes the image in the form of a raster of binary pixels is reproduced.
2. Discussion of Related Art
Devices whereby images are reproduced on the basis of digital image data are generally known. They are provided with inputting means for receiving the digital image data and with a reproduction unit which displays the pixels in their mutual relationship on a reproduction medium, such as a sheet of paper or a video display unit. The pixels in such cases are reproduced in one of two possible forms, e.g., white or black.
Problems arise with this reproduction if the boundaries of white and black image parts do not exactly agree with the separating lines between the pixels, because a pixel can only be reproduced in one form at a time. The boundaries between white and black image parts are thus imaged on the reproduction medium at the separating lines between the pixels. One effect of this is that boundary lines parallel to these separating lines are often shifted somewhat while on the other hand separating lines which extend at an angle over the image are reproduced as a stepped line. The skilled artisan knows this effect as "aliasing".
In many reproduction units the pixels have dimensions such that they can be distinguished separately. As a result, the above-described deviations are visible on the reproduction medium and result in reduced reproduction quality. An obvious step to improve reproduction quality in this respect is to increase the resolution of the pixel raster. Smaller pixels are less readily distinguished and, as a result, the aliasing effect is also reduced. Although the number of pixels, and hence, the number of processing operations, increases as a result with the square of the resolution increase, developments in electronics lead to ever-increasing processing speeds, so that the processing time can be kept within acceptable limits.
One disadvantage of this solution, however, is that a reproduction unit having a higher resolution is much more complex and expensive than a normal reproduction unit. However, this applies mainly in one direction. A reproduction image is built up of a raster of pixels which are displayed in successive (horizontal) lines. A laser printer, for example, contains a photoconductive medium which is charged and then exposed in successive lines by a beam of light modulated in accordance with the pixel values in the image data. Changing the distance between the lines is a far-reaching operation in the system and, hence, difficult and expensive. In another kind of printer, the photoconductive medium is exposed by light generated in an exposure head having LED's arranged in line. The distance between the LED's is a fixed specification of the printer and exposure heads with more LED's per mm are expensive.
In a magnetographic printer, the magnetizable surface of a drum is magnetized by a magnetic head array and then developed with magnetic toner. Such a printer is, e.g., described in U.S. Pat. No. 4,370,661. The density of the magnetic heads in the array fixes the resolution of the printed image. In an inductive printing device, such as the one described in U.S. Pat. No. 4,704,621, a rotatable process drum has a large number of parallel thin electrodes disposed on its surface, the electrodes extending in the direction of movement of the drum and being covered by a dielectric layer. A magnetic roller mounted adjacent the drum and parallel thereto develops the drum surface with toner where and when a voltage is applied to the electrodes. The resolution in the axis direction of the toner image thus formed is fixed by the density of the electrodes. This resolution, which is dictated by the apparatus construction, is henceforth termed the base resolution of the reproduction unit.